Xerographic copiers are now available in which the operator may choose between differently colored toners for developing the image. In some such copiers, it is possible to copy only in one color at a time, the color of the copy being determined by the color of the toner in a developer unit. Other such machines may contain two or more differently colored toners, selectable by the operator. Full color copiers, such as the Xerox (Trade Mark) 1005, are also known in which three or more separate images are automatically developed in succession with different colored toners to form a full color copy. Such full color copiers are, however, complex and expensive, and of limited utility in the ordinary office environment. It is, however, relatively simple to arrange for only selected portions of a copy to be developed in a color different from the rest of the copy. Thus, for example, it may be desired to copy a letter which has its text in black on a white background, but with a blue or red heading or logo. A simple xerographic copier can be relatively easily adapted to cater for this, by arranging for each copy to pass through the copying cycle twice, once using black toner, and developing all but the colored areas, and once using a colored toner, developing only the colored areas of the image. The relevant portions of the image to be copied in color may be selected manually by the operator, thereby setting the machine to cause selective erasure of the part of the image not to be developed on each cycle. One such machine which uses this 'color highlighting' approach is the Canon NP-3525.
It would be desirable to provide a way of detecting or recognizing color automatically, so that the making of `highlight color` copies as described above could be carried out automatically.
Color image reading apparatuses are known, for example, from U.S. Pat. No. 4,558,357, which describes a device having three lines of CCD arrays, in which each array is sensitive to a different color. As the image is scanned past the arrays, the outputs of the arrays are accumulated in line memories so that the color information from the three lines can be output in synchronism. U.S. Pat. No. 4,663,656 describes a CCD imager for a camera which uses four lines of CCD arrays, and a beam splitter to cause multiple line images of a single object line to fall on the arrays. Each array is made sensitive to a different color by means of a filter stripe. U.S. Pat. No. 4,675,727 describes a color document reading apparatus which uses a sensor array, such as a CCD array, with successive groups of three sensing elements along the array being sensitive to three different colors. Alternatively, three lines of arrays may be used, each array being sensitive to a different color. DE-A1-3,039,451 describes a device for electronically scanning pictures for reproduction on a television set. The device uses a two-line color-filtered CCD array with an external delay line of duration equal to the scanning interval between two rows. The two arrays do not, however, view corresponding areas of an image because the photosensitive elements for different colors are of different lengths. If such an arrangement were used for color detection or color recognition (such use not being described in the referenced document), it would give rise to false color detection because of the different areas viewed by the two lines. Of background interest are U.S. Pat. No. 4,761,683 to Matteson et al., which shows an arrangement for a color sensor array for imaging with three rows of sensors, each having a distinct color, U.S. Pat. No. 4,329,709 to Masuda et al., which describes a solid state imaging device with an arrangement of filtered photosites in a matrix providing the color signal information necessary within one horizontal period for the reduction of vertical smear. Additionally, U.S. Pat. No. 4,672,433 to Yamamoto et al. discusses the problems associated with detection of color by spacially displaced and differently filtered detectors. The patents cited herein are incorporated by reference. Additionally, the Toshiba TCD 140C-2 by Toshiba Corporation shows color detection by a series of arrays, each differently filtered, with the data subsequently stitched together.
Problems arise in colored text detection because of the (small) physical size of the lines which make up the letters of the text. In known color detection systems, as outlined above, color content comparisons have been made using three or more physically separated and filtered rows of sensors, or by using a single row of alternating red, green and blue filtered sensors. For alternating sensors, there must be several red, green and blue groups within the image of the line in order to prevent false color information. False color information can be obtained when each sensor is not looking at exactly the same area on a document. Thus a black line which falls on a red detector may not fall on a green or blue sensor. This gives an imbalance or false color information when the outputs of the sensors are compared.
This problem of false color leads to a requirement, using the known systems, either for very high resolution sensor arrays, or for accurately aligned multi-sensor array systems.